1. Field of the Invention
The present invention relates generally to electronic components suitable for receiving and retaining an electrical charge. The present invention relates more specifically to a charge storage device having a capacitance that is variable by alteration of the relative permittivity of the dielectric positioned between conductive electrodes within the device.
Applicant's novel device consists of two conductive plates sandwiching a conductive grid, typically embedded within a dielectric material. Charging the grid with a negative or positive potential changes the value of the dielectric constant (the relative permittivity of the dielectric) and thereby changes the capacitance of the device.
2. Description of the Related Art
Capacitors in general are important electrical/electronic components that are used in a variety of devices from basic power supply circuit boards to more complex computer systems. Capacitors are generally fabricated or constructed in two main forms, fixed and variable. A fixed capacitor has a preset capacitance that is established during the manufacture of the device through the selection of the dielectric material and the conductive plates that enclose the dielectric material. Variable, or trimmed capacitors, do not have set capacitance values fixed during their manufacture. Instead, variable capacitors are designed to allow a range of capacitance values by adjusting some feature of the capacitor to alter its capacitance value. Adjustment of a capacitor through its range of capacitance values may, for example, allow the fine tuning of an electronic circuit and various operational features of the circuit. Variable capacitors are therefore often utilized in electromagnetic wave transmitter and receiver circuitry to vary the frequency response for such transmitters and receivers.
Variable capacitors themselves come in a number of different structural and functional configurations. One of the most common methods of varying the capacitance is to interleave several movable plate electrodes among a number of fixed plate electrodes. Adjusting the position of the variable electrodes, relative to the fixed electrodes, increases or decreases the capacitance as the area between the electrodes changes. One problem with this type of variable capacitor is simply the number of electrodes that are required to implement the method of varying the capacitance. Multiple fixed and variable electrodes are required, and these electrodes themselves require a housing large enough to accommodate both them and their relative motion. In many instances, the size and geometry of such devices become unsuitable for small scale electronic applications. In addition, if the fixed and variable electrodes are not carefully structured and positioned, the capacitor may be easily damaged such that the geometry of the electrode plates changes in an undesired manner resulting in an inappropriate change in the capacitance.
In general, the capacitance value of a capacitor depends upon three factors. These include the distance between the electrode plates of the capacitor and the area (a two dimensional value) between the two electrodes or plates. A third factor not normally considered when constructing a variable capacitor is the relative permittivity of the dielectric material utilized. Most existing variable capacitors adjust either the distance between the two plates, and/or the area between the plates in order to adjust the capacitance value. In either case, mechanical motion is required in order to make these adjustments. It would be desirable if the capacitance value of a capacitor could be varied without the need for the mechanical motion of any of the components associated with the construction of the capacitor. Towards this end, the third factor involved in the capacitance value, the relative permittivity, may be examined as a basis for changing the capacitance value without requiring mechanical motion of the components.
The relative permittivity is, as mentioned above, also known as the dielectric constant, and is a relative measured value that depends on the material chosen for the dielectric. It is expressed as the ratio of a material's absolute permittivity to the absolute permittivity of a vacuum (see Equation 1 below).
In the field of electronics, capacitors are most often considered discrete electronic components that store electrical energy in the form of a static charge. A basic capacitor consists of two metal plates that are separated by a dielectric (insulator). One of the electrical properties of the dielectric insulator material is the ability to store a static electric charge. Capacitors are normally classified by the type of dielectric used in their construction (mica, ceramic, Mylar®, air, electrolytic, etc.) Each of the difference types of capacitors has a range of capacitance values that is generally determined by the geometry of the plates and the dielectric.
Once again to summarize, the capacitance value of a capacitor is the result of three variables:
A. the surface area of the two plates;
B. the distance between the two plates; and
C. the dielectric constant of the dielectric.
Capacitance values are measured in farads. Most fixed (non-variable) capacitors have a capacitance value between 1000 microfarads and 1 picofarad. There are, as mentioned above, a variety of variable capacitors known in the art. Existing variable capacitors operate on one of two principles, both of which require some form of mechanical movement. First, some variable capacitors change their capacitance value by changing their plate area. Second, some variable capacitors change their capacitance value by changing the distance between their plates.
Varactor or tuning diodes are also sometimes used as capacitors. A varactor or tuning diode is typically a semiconductor device that changes its capacitance by changing the width of its depletion region. Varactor diodes are typically limited to the picofarad range.
As mentioned above, capacitors are one of the most frequently used components in electronic circuits. One of the most common uses for variable capacitors is in tuning circuits. For example, the frequency tuner knob on a typical radio receiver is connected to a variable capacitor such that turning the knob changes the capacitance value of the capacitor, which changes the frequency of the radio signal that the radio receives. A variety of other uses of variable capacitors may be found in the literature that involve altering the characteristics of an RC circuit (a fundamental circuit component) by varying the capacitance value at some point in the circuit.
The voltage variable capacitor proposed herein is a modified form of existing capacitors. FIG. 1 shows in general how a conductive grid may be placed within a dielectric and connected to its own terminal. Charging the conductive grid with a negative potential causes the dielectric constant of the capacitor to decrease in value thereby reducing the capacitor's value. Placing a positive potential on the grid causes the dielectric constant to increase, thereby raising the capacitor's value.
The dielectric constant (or relative permittivity, εr) is a relative measure. It is expressed as the ratio of a material's absolute permittivity (ε) to the absolute permittivity of a vacuum (εo):εr=ε/εo  EQUATION 1
Total capacitance expressed in terms of the physical parameters of the capacitor may be expressed by the following equation:
                    C        =                              A            ⁢                                                  ⁢                                          ɛ                r                            ⁡                              (                                  8.85                  ×                                      10                    12                                    ⁢                                      F                    /                    m                                                  )                                              d                                    EQUATION        ⁢                                  ⁢        2            
As discussed above, existing variable capacitors vary the area (A) or the distance between the plates (d) in order to change the capacitance value (C). Both require a change in the physical parameters of the capacitor. In the present invention the relative permittivity (εr) is changed with a static charge on the conductive grid which changes the overall capacitance value without the requirement of any mechanical change in the capacitor. This is the principle by which the present invention operates. It can be seen therefore, from Equation 2, that an increase in the relative permittivity results in an increase in the capacitance value while a decrease in the relative permittivity results in a decrease in the capacitance value. It is known that altering the electromagnetic field within or surrounding a dielectric material will alter the relative permittivity of the dielectric. It is upon this principle that the present invention is based. As a practical matter, there are no limitations as to the size or geometry of the capacitor of the present invention or the type of dielectric material used.
Thus, the present invention provides a first plate capable of storing a charge therein in electrical contact with a first terminal of a source of power distribution. A second plate is spaced apart from the first plate and is in electrical contact with a second terminal of the source of power distribution. A dielectric material occupies the space between the plates as does the grid. The grid is attached to a means of charging, including variably charging the grid.
The grid of the present invention is typically comprised of a conductive material. The dielectric of the present invention is typically an insulator. Direct voltage or alternating voltage may be used as a means for charging the grid. The direct current may be variable. An exemplary method of using the capacitor of the present invention would be to vary the charge on the grid to achieve desired and selected electrical characteristics in a circuit. This may be done by varying the voltage, for example, to the grid and/or using a resistance in series with a power source and the grid.